The invention relates to coatings to be used on ceramic fiber insulation, and more particularly to such coatings which are resistant to severe thermal shock and chemical attack.
Ceramic fiber insulation is currently used as a high temperature lining for the interior walls and ceiling of furnaces and kilns. This insulation takes many forms, including blanket, felt, board, and modules with end strip and pleated configurations. Such materials have excellent insulating properties and are resistant to thernal shock, i.e. rapid changes in temperature.
However, ceramic fiber insulation is fragile and susceptible to corrosive attack by chemicals such as compounds of sodium, boron, and iron. This insulation is also susceptible to the erosive effects of fine particulate matter entrained in high velocity gases typically present in many furnace environments. Fuels such as natural gas, oil and coal produce carbon dioxide, carbon monoxide, water, hydrogen, hydrocarbons, carbon particles, and entrained ash particles. To meet the need for protection of ceramic fiber insulation, various coatings have been developed. These coatings are typically blends of ground ceramic fiber and binders. U.S. Pat. No. 3,231,401 (Price et al) shows a combination of a ceramic fiber with an aqueous dispersion of colloidal inorganic oxide to produce a thermally shock resistant refractory coating or adhesive useful to about 2300.degree. F. By making the coating out of the material to be coated, the problem of thermally matching coating to substrate (i.e. the surface of the underlying ceramic fiber insulation) is essentially eliminated but at great economic expense.
A major disadvantage of ceramic fiber-based coatings is their high cost. Ceramic fiber if an expensive material, and these coatings are dense and therefore use a relatively great quantity of fibers.
Additionally, these coatings generally fail to satisfactorily meet the requirements of a protective coating: good adhesion to the substrate and resistance to severe thermal shock combined with good chemical resistance. These coatings often crack and peel off the substrate, especially under thermal shock conditions.
Many of the problems associated with the prior art are overcome to a substantial extent by the practice of the invention.